Process for the manufacture of crystalline vinylidene chloride polymer tubing whose walls do not cohere



July 12, 1949. K, G, FRANCls 7 2,476,140

PROCESS FOR THE MANUFACTURE OF CRYSTALLINE VINYLIDENE CHLORIDE POLYMERTUBING WHOSE WALLS DO NOT COHERE Filed Aug. 11, 1948 INVEN TOR. Ken/762%G. Franc/L9 ATTORNEYS Patented July 12, 1949 PROCESS FOR THE MANUFACTURECRYSTALLINE VINYLIDENE CHLORIDE POLYMER TUBING WHOSE WALLS DO NOT COHEREKenneth G. Francis, Bay City, Mich, assignor to The Dow ChemicalCompany, Midland, Mich., a corporation of Delaware Application August11, 1948, Serial No. 43,644

This invention relates to an improvement in the process for making filmtubes from normally crystalline vinylidene chloride polymers.

The principal method employed for producing film from normallycrystalline polymers of vinylidene chloride has been described by W.Stephenson in his copending application, Serial No. 721,738, filedJanuary 13, 1947, now U. S. Patent No. 2,452,080. So far as is herepertinent, that method comprises fusion of the polymer or copolymer,extrusion thereof in tubular form downwardly into a bath of cold waterto supercool it while maintaining a column of inert lubricating liquidin that portion of the freshly extruded tube which passes verticallydownward into the said bath, conducting the supercooled tube out of thebath and between spaced pairs of pinch rolls, and inflating that portionof the tube between said pinch rolls with such an amount of air that theaddition of more air results in no further increase in diameter of thetube but only in elongation of the bubble and migration of thestretching zone away from the second set and toward the first set ofpinch rolls. Said method is applicable not only to the polymer ofvinylidene chloride alone but also to those vinylidene chloridecopolymers which are normally crystalline, as determined by X-raydiffraction studies. It applies to extrudable compositions of suchnormally crystalline polymers, whether plasticized or unplasticized. Allsuch bodies are referred to herein as normally crystalline vinylidenechloride polymers.

One problem which has arisen from the use of the above-described priormethod has been due to the tendency of the fully stretched andrecrystallized film tube, which becomes flattened in passing through thesecond set of pinch rolls, to resist reopening to tubular form, due tocoherence between opposed internal faces of the flattened tube. Since,for most uses of the film, it is necessary to open the tube prior tofilling sections thereof with merchandise or prior to slitting it toform a single film sheet, it is desirable to avoid or overcome itsresistance to being opened. The stated problem is particularly acutewhen handling films made from the copolymer of about 73 per centvinylidene chloride and 27 per cent vinyl chloride, and is overcome bythe hereinafter described invention even with such a film.

It is the principal object of the present invention to provide animprovement in the stated process which will yield the film tube in areadily opened form, i. e. one which will overcome the stated tendencyfor opposed internal walls of the 2 Claims. (Cl. 1847.5)

flattened film tube to stick together. A related object is to provide atreatment for the internal walls of a tube of normally crystallinevinylidene chloride polymer, during the course of its manufacture, sothat said tendency toward coherence is overcome. A further object is toprovide such a treatment, wherein the agent employed to produce thedesired result is one which will not interfere with future welding orheat-sealing operations carried out on such film, and will not weakenthe resulting welds or seals.

I have found that the foregoing desired results are obtained when, inaccordance with the method described in connection with Fig. 4 of saidStephenson copending application (after which is patterned the drawingherein), the supercooled tube is conducted vertically upward from alower set of pinch rolls to a higher set, stretching to effectrecrystallization occurs between the said sets of rolls, and, accordingto the present improvement, finely powdered crystalline vinylidenechloride polymer, of composition at least similar to that of the tube,is brought into contact with the internal walls of the tube between saidsets of pinch rolls. The polymer powder may be in the form of a loosepile carried within the tube above the nip of the lower set of rolls,or, preferably, it may be contained within a dusting bag carried at thesame position within the tube, as illustrated, or carried on theshoulders of the tube in stretching zone A. The dusting bag, when used,may be made of any cloth sufficiently porous to exude or sift out thefine polymer powder continuously as the tube moves upward past andaround the bag. The use of finely ground polymer, as described, preventsthe inner tube walls from sticking together when the tube issubsequently collapsed, and the minute coating of polymer powder thereonin no way interferes with subsequent welding or heat-sealing operations,as the powder is of nearly the same composition as the film and becomesan integral part of any seal or weld. It is preferred to use a polymericpowder in the invention having a particle size which will pass a meshscreen, U. S. sieve series, and the finer the powder the more successfulis the treatment. The best particle sizes yet tried are those of a fewmicrons or finer. An 8 to 12 ounce supply of polymer powder placedwithin the tube over the lower pinch rolls is found to last for severalhours, and to prevent completely the previously observed and troublesomesticking tendency in the subsequently flattened tube.

The single figure of the annexed drawing illustrates the preferredpractice of the invention. A

2,47 0,140 3 4 normally crystalline vinylidene chloride polymer wardbetween two spaced pairs of pinch rolls, is fused and extrudeddownwardly through a and inflating the tube to efiect stretching andtube-forming orifice of a plastics extruder l0. recrystallizationthereof between said sets of rolls The freshly formed tube ll fallsdirectly into a by introducing air thereto under pressure until bath ofcold water "r2 -w-herein- -*it is supercoo ed; 5 the-introduction -ofmores'airgrresults in no further It is passed through asubme'itgd set ofpinch "radial expansioh of the tube and causes only rolls [3, or otherconstrictin m an and a c lelongation of the inflated portion of the tubeumn of oil l4 or similar inert lu ric in l quidi toward the lower pinchrolls, the improvement is Ca r ed in the Portion of the be @DOY? 110 2whichconsists in. maintaining Within the tube The flattened and li qd liift T abov'the ldwerp'irich rolls a body of finely powarollnd guider011 l5 a t nw P21 56; .(ierelslcrystalli e vinylidene chloride polymerof bath I2 and betw n tw s ts ofp i ,"cciiipsiticnk s-imiia to that ofthe tube, in effecand IT located One Set above the other, as tivedusting contact with the inner walls of the When Starting Operat o t tb-$ QDTQQ an v hrebytopvfcome the normal tendency a dusting ba 3 fu offinelyiiividdicliysta-llififil? thbse walls tb stick to one another whenthe vinylidene chloride polymer of composition similar" thb i qigtfi iai passing through the upper to that of the tube is droppedintotlre-tube,conrpinchrolls, i ing to rest above the nip between rolls [6. The 2'.In'a niethod bf making film in tubular form tube is inflated with air toeffect radial distension fmmra: normally crystalline vinylidene chlorideto from 3 to 5 times the fsupercooled diameter and 20 polymer whichcomprises forming a supercooled until the addition of more air-results.merely in tube of such.polymer, conveying it vertically up- -.migrationof stretching z'one YA doysrnvmrd toldlloetweenftwospaced"pairs ofpinch'rolls, and ward rolls is. The fully s'tretched tube is; r- H mifect'stifetching ahdre rys; warded in a flattened conditionbyrollsfl'lg which .tallizatiohfthereof between. said sets of rolls jb'yoperate at a peripheralrspeed offrom 2to 4 times 2 ',introducihg air"thereto under pressure "until the that of rolls [3, J5, and 1.5-, andistakenv up "0:151 introduction 'ofjinore fair results in no further"storage reel I9. Eyfen'afterbeingYtightIy wrapped i' if adialezipahsion of th Ltube and causes onlyonreel.'l.9-orprolongedperio'ds'it is fou'ri'dithalt elongation "ofl'the in ted portion of the, tube the tube may beopenedeasilyfwhereasfatiibe fowardlthe lower binel i"rolls ,.the improvementso-stored withoutthe intern l treatment with.. f tviiichgitinsists inmaintainin within the tube polymer powder offers-the greatestrs'istancefto -a bofyethe"lowerjoirifcli 'rblls abody of crystallinebeing opened. H p fyiriylideneliloride'polymer of nOmpoSitionsimiIa r Bythe term polymer of similar 'Tc'ompositidhfl foithat'of; the tube and ofa particlesizie to pass as herein employed, is meant 'acrystallinepolyfa 'IfiO' 'e'sh screen, sivfefserie s, contained merofvinylidene chloride which is. the same @3 nlfa p0 Otis bag throughwhichthepowderedpolythat from which thefilm tube. is made, or, ii of"riiiaysift intojcdntactwith-the inner wallsdf differentpolymericcomposition,Ihaslafusion tom;- the tube, therebyjto 'oyercomethe normal ten- Lperature'within 10 centigrade degrees "of that/of d'efidy'ofthus'e wangjt tieg t g an t hen l. 'Inafrnethod'ofjinakingfilm intubular form KENNETH G. FRANCIS.

from a normally crystalline. vinylidene chloride V polymer whichcomprises forming 'a supercooled No references cited.

tube er such polymer, o nueyi'ng it vertically 11

